Systems And Methods For Allocating Timeslots For Unicast Transmissions

ABSTRACT

Systems and methods are described for reducing an end-to-end latency occurring in link layer unicast transmissions by shifting the timeslots in the super frame of the TDMA mode based on membership to a multicast group. In this manner, transmission to all of the multicast subscribers occupy adjacent timeslots, which results in an optimization of slot allocation and improvement of the end-to-end latency for multicast communication.

This application claims priority to U.S. provisional patent applicationhaving Ser. No. 62/880,355 filed on Jul. 30, 2019. This and all otherreferenced extrinsic materials are incorporated herein by reference intheir entirety. Where a definition or use of a term in a reference thatis incorporated by reference is inconsistent or contrary to thedefinition of that term provided herein, the definition of that termprovided herein is deemed to be controlling.

FIELD OF THE INVENTION

The field of the invention is multicast communication, such as for usein intra-vehicular wireless sensor networks, Internet of Things, homeautomation, and low-power devices, for example.

BACKGROUND

The following description includes information that may be useful inunderstanding the present invention. It is not an admission that any ofthe information provided herein is prior art or relevant to thepresently claimed invention, or that any publication specifically orimplicitly referenced is prior art.

Currently, there is an increasing demand for new applications ofwireless sensor networks (WSNs). One exemplary area of applicationconcerns in-vehicle communications, especially for aircrafts. Asaircraft systems are becoming more and more connected, additionalsensors are being employed in the aircraft to determine a status andhealth of seats, electronic equipment, and other components. With theseincreasing demands for new applications, it is useful to connect WSNs tothe Internet via an Internet Protocol (IP) addressing, and specificallyvia Internet Protocol version 6 (IPv6) and future protocols, to achieveall-IP communication and thus, achieve interoperability.

However, current standards including Institute of Electrical andElectronics Engineers (IEEE) 802.15.4 do not natively support multicast.Instead, algorithms have been developed that utilize link layer unicasttransmission or link layer broadcast transmission to achieve IPmulticast. While link layer unicast transmissions offer high reliabilitydue to the re-transmissions based on acknowledgment, they have poorend-to-end latency as individual transmissions are required to each ofthe multicast subscribers and require high energy consumption. Incontrast, broadcast transmissions have low end-to-end latency and lowenergy consumption due to a single transmission to all multicastsubscribers, but there is lower reliability because of the lack ofre-transmissions and acknowledgements.

Thus, while various techniques have been developed to attempt to addressthe problems identified above, each has its own problems.

For example, the IEEE 802.11aa standard identifies various mechanismsfor multicast transmission. A study of the standard can be found in “Astudy of IEEE 802.11aa,” by B. T. Vijay and B. Malarkodi, published inthe 2017 8th International Conference on Computing, Communication andNetworking Technologies (ICCCNT), Delhi, in 2017. The standard addressesmethods of transmission of stream (e.g., voice, audio, and video) overmulticast communication.

Using this standard, the coordination between multiple access points(APs) can be improved by adding new control messages and mechanisms. Inaddition, wireless network performance can be better assessed withinterferences more likely avoided with better channel selection. Loadsharing can also be accomplished among APs, through additional ways toschedule and add broadcast transmission opportunities between APs. Usinga Group Cast with Retries (GCR) service, multicast frames are sentseveral times without awaiting any acknowledgement, which leads toincreased delivery probability.

However, the 802.11aa standard is a non-deterministic scheme based onCSMA and the GCR service adds overhead depending on the number ofretries and becomes inefficient and unnecessary when the link quality isgood.

As another example, the TRM-MAC protocol can be used to manage availableslots to a current node density. In case of a low node density, minimumslots can be used. For a large node density, additional slots can beadded. The TRM-MAC protocol can result in a higher successfultransmission rate with a minimum delay, by providing a trade-off betweenenergy and delay. Additional details about the protocol can be found inthe article “TRM-MAC: A TDMA-based reliable multicast MAC protocol forWSNs with flexibility to trade-off between latency and reliability” byBhatia and Hansdah, published in 2016 in Computer Networks. As shown inFIG. 1, the MAC-frame is subdivided into three parts, acontention-free-period-1 (CFP1), a contention-free-period-2 (CFP2) and acontention-access-period (CAP).

The channel access mechanism employed in the CFP1 of the MAC-frame isTDMA, and it is used by the internal nodes to relay multicast data. Theslot-size in this portion is typically equal to the time required totransmit multicast data (at the current data rate, t_(data)). The numberof slots in the CFP1 portion depends upon the number of internal nodesin the Multicast Spanning Tree (MST) and the algorithm used to performthe TDMA slot-scheduling for the internal nodes.

The channel access mechanism employed in the CFP2 of the MAC-frame isalso TDMA and is used by the leaf nodes using ACK-based approach totransmit their acknowledgement (ACK) messages. The slot size in thisportion is typically equal to the time required to transmit an ACKmessage (tack) and is usually smaller than the slot size in CFP1. Thenumber of slots in this portion again depends upon the number of leafnodes using ACK-based approach in the MST and the algorithm used toperform the TDMA slot-scheduling.

The channel access mechanism employed in the CAP portion of theMAC-frame is prioritized-CSMA, in which a child node with local id, j,using NACK-based approach transmits a NACK message in the CAP portion ofMAC-frame, provided (a) it finds the channel idle for a duration definedby j−1−nRouters−nACK)*t_(cca) and (b) it has not received the data. Thet_(cca) is the time required to perform clear channel assessment (CCA)by any sensor node to ensure that no other node is already transmitting,and n Routers is the number of internal-child nodes of its parent. Thevalue of t_(cca) is usually smaller than t_(data), t_(ack) and t_(nack).The reason for calling this mechanism as prioritized-CSMA is that thenodes with smaller local ID value have higher priority to access thechannel as compared to the nodes with larger local ID value. The lengthof this portion would be (α−1)*t_(cca)+t_(nack), where α is the maximumnumber of leaf-child nodes using NACK-based approach at any internalnode, in the network.

TRM-MAC can be disadvantageous because it does not propose a way tooptimize slots allocation in the CFP. In addition, the CCA operation mayfail in the presence of hidden nodes, and therefore, the proposedprioritized-CSMA channel access mechanism does not completely avoid thecollision between NACK messages.

As another example, optimization-based rate allocation and scheduling inTDMA-based wireless mesh networks can be used which is further describedin the article “Optimization based rate allocation and scheduling inTDMA based wireless mesh networks” by Wang, Mutka, and Torng, publishedin the 2008 IEEE International Conference on Network Protocols. In thisarticle, the authors worked to optimize the multicast and unicastcommunications of a TDMA protocol considering delay and throughput. Toaccomplish this result, they introduced a multi-transfer rate schedulingalgorithm based on the construction of a perfect graph. It firstconstructs a spanning tree rooted at the gateway, then prunes the treeto accommodate all sessions to achieve the perfect graph.

Unfortunately, the algorithm uses broadcast slots for its multicasttransmission, which is unreliable. It requires centralized control andcan reduce special reuse. In addition, the algorithm is quite complex toput in place and requires some computational power. The algorithm mayalso require bigger timeslots adding latency to the overall slot frame.

All publications identified herein are incorporated by reference to thesame extent as if each individual publication or patent application werespecifically and individually indicated to be incorporated by reference.Where a definition or use of a term in an incorporated reference isinconsistent or contrary to the definition of that term provided herein,the definition of that term provided herein applies and the definitionof that term in the reference does not apply.

Thus, there is still a need for reducing end-to-end latency in unicasttransmissions.

SUMMARY OF THE INVENTION

The inventive subject matter provides apparatus, systems and methods forreducing end-to-end latency in a low-power wireless personal areanetwork (6LoWPAN) using a link layer unicast mode. In such networks, theprotocol in the link layer (L2) is typically IEEE 802.15.4, and theprotocol in the network layer (L3) adopts IPv6.

IEEE 802.15.4 provides a low-power energy-efficient protocol. The IEEE802.15.4 protocol has a broad scope of applications including, forexample, home automation, factory automation, predictive maintenance aswell as live monitoring. IEEE 802.15.4 can also become deterministicwhen using one of its Time Division Multiple Access (TDMA) modes, wherethe communication between nodes is divided into timeslots and scheduledon a super frame (SF). However, any TDMA based protocol could be used.

It should to be understood that, while the systems and methods disclosedherein are described in connection with the IEEE 802.15.4 protocol, itis contemplated that the systems and methods disclosed herein could beused with later-developed protocols where reallocation of transmissionsto subscribers of a multicast group can reduce end-to-end latency.

Latency in such networks can be reduced by shifting timeslots in thesuper frame when in one of the TDMA modes. The reallocation of timeslotsis preferably based on membership to a multicast group, such thattransmission to each of the multicast subscribers of a group occupiesadjacent timeslots. This can advantageously result in an optimizedtimeslot allocation and improvement of the end-to-end latency formulticast communication.

The inventive subject matter described herein has various applications.In an aircraft setting, for example, in-vehicle sensors could be dividedinto separate multicast groups. In such an environment, galleys couldform a first multicast group, each seat class could form separatemulticast groups, and on-board trolleys could be formed as separatemulticast groups.

Multicast groups could also be formed based on applications. Forexample, oxygen mask release at the passenger service unit could form aseparate group.

Various objects, features, aspects and advantages of the inventivesubject matter will become more apparent from the following detaileddescription of preferred embodiments, along with the accompanyingdrawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art diagram illustrating the TRM-MACprotocol.

FIG. 2 illustrates a flow chart for a smart broadcast forwardingalgorithm.

FIG. 3 illustrates additional flow charts for the smart broadcastforwarding algorithm.

FIG. 4 illustrates a diagram showing dynamic allocation of timeslots ina super frame.

FIG. 5 illustrates a diagram showing the shifting of timeslots in thesuper frame.

FIG. 6 illustrates one embodiment of a method for reducing end-to-endlatency.

DETAILED DESCRIPTION

Throughout the following discussion, references may be made regardingaccess points, routers, servers, services, interfaces, portals,platforms, or other systems formed from computing devices. It should beappreciated that the use of such terms is deemed to represent one ormore computing devices having at least one processor configured toexecute software instructions stored on a computer readable tangible,non-transitory medium. For example, an access point can be configured tofulfill preprogrammed roles, responsibilities, or functions.

The following discussion provides many example embodiments of theinventive subject matter. Although each embodiment represents a singlecombination of inventive elements, the inventive subject matter isconsidered to include all possible combinations of the disclosedelements. Thus if one embodiment comprises elements A, B, and C, and asecond embodiment comprises elements B and D, then the inventive subjectmatter is also considered to include other remaining combinations of A,B, C, or D, even if not explicitly disclosed.

FIGS. 2 and 3 illustrate flow charts for one embodiment of a smartbroadcast algorithm. Such concepts are further described in co-pendingpatent application no. PCT/US20/44318 filed on Jul. 30, 2020. As shown,the algorithm utilizes both link layer unicast and broadcasttransmissions and introduces bidirectionality in multicastcommunication. In this manner, the multicast packets can traverse bothup and down an RPL tree illustrating a routing protocol for a low powernetwork, and the multicast packet does not have to traverse all the wayto the root.

FIG. 2 illustrates an exemplary process for forwarding packets. Asshown, a mote receives a packet whose destination is a multicast groupaddress, and it is determined whether the packet was received from apreferred parent.

If the packet was received from a preferred parent, it is checkedwhether the multicast group is listed in the routing table. If so, thepacket is forwarded downward using a link layer unicast or broadcasttransmission based on the number of children and then it is checkedwhether the node (node and mote are used interchangeably herein torepresent devices in the wireless sensor network) is a member of themulticast group. If the multicast group is not listed in the routingtable, the packet is not forwarded, and it is checked whether the moteis a member of the multicast group.

If the mote is a member of the multicast group, the packet is deliveredup the network stack and the packet is accepted. If not, the packet isdropped.

If the packet was not received from a preferred parent, it is checkedwhether the packet arrived from below in the RPL tree (i.e. receivedfrom a link layer unicast transmission). If yes, it is checked whetherthe multicast group is listed in the routing table. If not, the packetis dropped.

If the multicast group is listed in the routing table, the packet isforwarded downward using link layer unicast, except to the child fromwhich the packet was received. It is then checked whether the mote isthe RPL root. If so, it is checked if the mote is a member of themulticast group. If not, the packet is forwarded upward to the preferredparent.

If the mote is a member of the multicast group, the packet is deliveredup the network stack and the packet is accepted. If not, the packet isdropped.

Because all the mote (sensors) in the network are aware of the topology,a multicast source can be anywhere within the network. The processor candetermine whether to utilize unicast or broadcast at the link layerbased on the number of interested children and the duty cycle rate.

To improve the reliability of messages being received during broadcasttransmissions, time slotted channel hopping (TSCH) can be used toprovide for broadcast retransmissions. As shown in FIG. 3, a packet canbe retransmitted a predefined number of times. Because noacknowledgements will be received, this retransmission will alwaysoccur. The timeslots in TSCH can be classified according tofunctionality so that there are timeslots for transmitting and receivingbroadcast retransmissions. To reduce energy requirements of recipientsduring these retransmissions, any remaining timeslots where thebroadcast retransmissions can be received are set to sleep mode once apacket is successfully received. Thus, the recipient mote will entersleep mode after receiving the packet and subsequent retransmissionswill be ignored. Thus, the probability of a packet being received can beincreased while reducing the energy requirements of the recipients.

For unicast transmission, as the number of subscribers increases, thenumber of radio transmissions increase due to the individualtransmission of packets to every subscriber as well as theacknowledgements being sent back. Because the timeslots for multicastcommunication can be spread across the super frame, end-to-end latencycan be increased.

This is at least partly caused by the inherent delay introduced due tothe TDMA mode. For example, FIG. 4 illustrates an exemplary timeslotschedule at mote B. As shown in the first super frame (SF1), thetimeslot for transmission from mote B to subscriber 1 is followed by atimeslot for reception from subscriber 1, a timeslot for transmissionfrom mote B to its parent mote A, a timeslot for reception from mote A,and a timeslot for transmission from mote B to subscriber 2. Assumingthe duration of each timeslot is 10 milliseconds, the transmission tosubscriber 2 occurs three timeslots after the transmission to subscriber1 or 30 milliseconds after the first transmission. These 30 millisecondsare added on to the end-to-end latency. Hence, as the number ofsubscribers increases, the end-to-end latency may also increase.

To reduce such latency, preferred systems and methods shift thetimeslots in the Super Frame based on membership to a multicast group.In this manner, all the multicast subscribers can occupy adjacenttimeslots, which results in an optimization of the allocation of slotsand reduces the end-to-end latency for multicast communications.

An example of this is shown in FIG. 5. As shown in the Figure, thetimeslots in the super frame can be shifted (re-allocated) based onmembership of the subscribers (motes) to a multicast group sotransmissions to the multicast subscribers occupy adjacent timeslots. Inthis example, motes 1 and 2 are subscribers to the multicast group, sothe Slot 5 transmission is shifted to Slot 2, such that it is adjacentto the transmissions from mote B are in adjacent timeslots. This wouldresult in a minimum of 30 milliseconds saved in end-to-end latency.Where the number of subscribers is high such as in a real-worldenvironment like an aircraft, the end-to-end latency can be reduceddrastically.

FIG. 6 illustrates an exemplary method 600 in accordance with disclosedembodiments for reducing end-to-end latency in link layer unicasttransmissions using the IEEE 802.15.4 protocol. In step 610, a routercan be provided having a processor and memory. In some embodiments, therouter is a component of an access point (AP) in the network.

In step 620, timeslots of a time division multiple access (TDMA) superframe can be dynamically allocated by the router for transmission to andacknowledgements from a set of motes. The set of motes can comprise aplurality of motes that includes a first motes, a second motes, and athird motes. Transmission to each of the motes from mote A can therebybe allocated a distinct timeslot.

In step 630, the router or other component can determine whether any ofthe motes of the set are members of a multicast group.

If determined that the second mote is a member of the same multicastgroup as the first mote, the timeslot allocated for transmission to thesecond mote can be shifted or reallocated in step 640 such that thetimeslots for transmission to the first and second motes are adjacent.

If determined that the third mote is also a member of the same multicastgroup as the first mote, the timeslot assigned for transmission to thethird mote can be shifted or reallocated in step 650 such that thetimeslots for transmission to the second and third motes are adjacent.

In one aspect, a system for reducing end-to-end latency in in link layerunicast transmissions using the IEEE 802.15.4 protocol comprise a routerhaving a processor and a control thread stored on a non-transitorycomputer readable medium. It is contemplated that the processor andcontrol thread can enable a transceiver module at a beginning of atimeslot of a TDMA super frame. Although IEEE 802.15.4 protocol ismentioned above, any TDMA based protocol could be used.

The processor can be configured to dynamically allocate a timeslot fortransmission to each of a set of motes. The processor can then determineif each of the motes of the set is a member of a first multicast group.If a mote of the set is a member of the first multicast group, therouter can shift the timeslot for transmission to that mote, such thatthe shifted timeslot is adjacent to another of the timeslots allocatedfor transmission to a subscriber of the first multicast group. Thisprocess can continue until timeslots allocated for transmission to allthe motes of the set that are members of the first multicast group areadjacent to another of those timeslots.

In some contemplated embodiments, the processor can be configured todetermine whether each of the motes of the set is a member of a secondmulticast group. The processor can then shift timeslots collocated fortransmission to each mote that is a member of the second multicastgroup, so that each timeslot for transmission to each of the members ofthe second multicast group are shifted to be adjacent to another of thetimeslots allocated for transmission to members of the second multicastgroup.

Preferably, the processor defines a structure of the super frame basedon IEEE 802.15.4 according to a topology of a wireless sensor networkcomprising the set of motes.

The systems and methods described herein can be further embodied in aprogram stored in non-transitory computer-readable storage medium.Preferred programs are configured to execute a set of operations formulticast communication over an IP network when the program is executedby one or more processors of a router.

Contemplated operations include dynamically allocating timeslots of aTDMA super frame for transmission from a first mote to a second mote andfrom the first mote to a third mote, where each transmission isallocated a distinct timeslot. For example, a transmission from thefirst mote to the second mote can be allocated a first timeslot.

Another operation can determine if the third mote is a member of thesame multicast group as the second mote. If so, a timeslot allocated fortransmission to the third mote can be relocated or shifted based on amembership of the third mote to the multicast group, such that thereallocated second timeslot is adjacent to the first timeslot.

Similarly, a third timeslot of the TDMA super frame can be dynamicallyallocated for transmission from the first mote to a fourth mote.

Another operation can determine if the fourth mote is a member of thesame multicast group as the second mote. If so, a timeslot allocated fortransmission to the fourth mote can be relocated or shifted based on amembership of the fourth mote to the multicast group, such that thereallocated third timeslot is adjacent to the second timeslot.

Preferably, the motes wirelessly communicate over a low-power wirelesspersonal area network using the IEEE 802.15.4 protocol.

Although a central entity can distribute a fixed schedule (i.e., a listof timeslots of the super frame) to each node within the network toimprove determinism, the inventive concepts described herein addresstimeslot allocation in environments where timeslots are dynamicallyallocated by the nodes based on their distance between each other. Insuch environments, timeslots may be allocated randomly, and may not beoptimized for end-to-end communication, and more specifically, formulticast communication where the nodes use 802.15.4 unicasttransmission to send multicast packets.

As used herein, and unless the context dictates otherwise, the term“coupled to” is intended to include both direct coupling (in which twoelements that are coupled to each other contact each other) and indirectcoupling (in which at least one additional element is located betweenthe two elements). Therefore, the terms “coupled to” and “coupled with”are used synonymously.

In some embodiments, the numbers expressing quantities of ingredients,properties such as concentration, reaction conditions, and so forth,used to describe and claim certain embodiments of the invention are tobe understood as being modified in some instances by the term “about.”Accordingly, in some embodiments, the numerical parameters set forth inthe written description and attached claims are approximations that canvary depending upon the desired properties sought to be obtained by aparticular embodiment. In some embodiments, the numerical parametersshould be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques. Notwithstandingthat the numerical ranges and parameters setting forth the broad scopeof some embodiments of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspracticable. The numerical values presented in some embodiments of theinvention may contain certain errors necessarily resulting from thestandard deviation found in their respective testing measurements.

Unless the context dictates the contrary, all ranges set forth hereinshould be interpreted as being inclusive of their endpoints andopen-ended ranges should be interpreted to include only commerciallypractical values. Similarly, all lists of values should be considered asinclusive of intermediate values unless the context indicates thecontrary.

As used in the description herein and throughout the claims that follow,the meaning of “a,” “an,” and “the” includes plural reference unless thecontext clearly dictates otherwise. Also, as used in the descriptionherein, the meaning of “in” includes “in” and “on” unless the contextclearly dictates otherwise.

The recitation of ranges of values herein is merely intended to serve asa shorthand method of referring individually to each separate valuefalling within the range. Unless otherwise indicated herein, eachindividual value with a range is incorporated into the specification asif it were individually recited herein. All methods described herein canbe performed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context. The use of any and allexamples, or exemplary language (e.g. “such as”) provided with respectto certain embodiments herein is intended merely to better illuminatethe invention and does not pose a limitation on the scope of theinvention otherwise claimed. No language in the specification should beconstrued as indicating any non-claimed element essential to thepractice of the invention.

Groupings of alternative elements or embodiments of the inventiondisclosed herein are not to be construed as limitations. Each groupmember can be referred to and claimed individually or in any combinationwith other members of the group or other elements found herein. One ormore members of a group can be included in, or deleted from, a group forreasons of convenience and/or patentability. When any such inclusion ordeletion occurs, the specification is herein deemed to contain the groupas modified thus fulfilling the written description of all Markushgroups used in the appended claims.

It should be apparent to those skilled in the art that many moremodifications besides those already described are possible withoutdeparting from the inventive concepts herein. The inventive subjectmatter, therefore, is not to be restricted except in the spirit of theappended claims. Moreover, in interpreting both the specification andthe claims, all terms should be interpreted in the broadest possiblemanner consistent with the context. In particular, the terms “comprises”and “comprising” should be interpreted as referring to elements,components, or steps in a non-exclusive manner, indicating that thereferenced elements, components, or steps may be present, or utilized,or combined with other elements, components, or steps that are notexpressly referenced. Where the specification claims refers to at leastone of something selected from the group consisting of A, B, C . . . andN, the text should be interpreted as requiring only one element from thegroup, not A plus N, or B plus N, etc.

What is claimed is:
 1. A method for reducing end-to-end latency usinglink layer unicast transmission in a network, comprising: providing arouter having a processor and memory, wherein the role of the router canbe taken up by any mote that is part of the network topology;dynamically allocating timeslots of a time division multiple access(TDMA) super frame for transmission from a mote A to a set of motes,such that transmission to each of the motes is allocated a distincttimeslot, wherein the set of motes comprises a first mote, a secondmote, and a third mote; wherein the first mote, second mote and thirdmote are each members of a first multicast group; wherein mote A isallocated a first timeslot for transmission to the first mote; therouter reallocating a second timeslot for transmission from mote A tothe second mote based on a membership of the second mote to the firstmulticast group, wherein the reallocated second timeslot is shifted suchthat the second timeslot is adjacent to the first timeslot; and therouter reallocating a third timeslot for transmission from mote A to thethird mote based on a membership of the third mote to the firstmulticast group, wherein the reallocated third timeslot is shifted suchthat the third timeslot is adjacent to the second timeslot.
 2. Themethod of claim 1, wherein the router wirelessly communicates using aTDMA (Time Division Multiple Access) based protocol.
 3. The method ofclaim 1, wherein the router dynamically allocates the timeslots andreallocates the second and third timeslots.
 4. The method of claim 1,wherein the router wirelessly communicates over a low-power wirelesspersonal area network.
 5. The method of claim 1, further comprising: therouter receiving a packet; determining whether the packet has amulticast group address that is a preferred parent using the processor;if the origin is the preferred parent, the router checking whether themulticast group address is listed in a routing table of the router; ifthe origin is not the preferred parent, dropping the packet; only if themulticast group address is listed in the routing table, the routerforwarding the packet downward using the link layer unicast transmissionas a function of the number of children; after determining whether themulticast group address is listed in the routing table, the routerchecking whether the first node is a member of the multicast group; andif the router determines that the first node is the member of themulticast group, delivering the packet upward.
 6. The method of claim 5,further comprising: if the origin is not the preferred parent, therouter checking whether the packet was received from below; if therouter determines the packet was received from below, the routerdetermining whether the multicast group address is listed in the routingtable; if the router determines the packet was not received from below,dropping the packet; and if the multicast group address is listed in therouting table, the router forwarding the packet downward using a linklayer unicast transmission, but not forwarding the packet to the childfrom which the packet was received.
 7. A router, comprising: aprocessor; and a control thread stored on a non-transitory computerreadable medium; wherein the processor and control thread enable atransceiver module at a beginning of a slot of a time division multipleaccess (TDMA) super frame; wherein the processor dynamically allocates atimeslot for transmission to each of a set of motes for link layerunicast transmission using a first protocol; wherein the processordetermines whether each of the motes of the set is a member of a firstmulticast group; and the router shifting an allocated timeslot fortransmission to each of the motes of the set that is a member of thefirst multicast group, such that each timeslot for transmission to eachof the members of the first multicast group is shifted to be adjacent toanother of the timeslots.
 8. The router of claim 7, further comprising:wherein the processor determines whether each of the motes of the set isa member of a second multicast group; and the router shifting anallocated timeslot for transmission to each of the motes of the set thatis a member of the second multicast group, such that each timeslot fortransmission to each of the members of the second multicast group areshifted to be adjacent to another of the timeslots allocated fortransmission to members of the second multicast group.
 9. The router ofclaim 7, wherein the first protocol comprises an IEEE 802.15.4 protocol.10. The router of claim 7, wherein the processor defines a structure ofthe super frame based on IEEE 802.15.4 according to a topology of awireless sensor network comprising the set of motes.
 11. The router ofclaim 7, further comprising: determining whether each of the motes ofthe set has data to send by timestamping each packet received from afirst sensor during the allocated one of the TDMA super frame timeslots.12. The router of claim 7, wherein the router wirelessly communicatesover a low-power wireless personal area network.
 13. A program stored innon-transitory computer-readable storage medium, wherein the programexecutes the following operations for multicast communication over an IPnetwork when the program is executed by one or more processors of arouter, the operations comprising: dynamically allocating timeslots of atime division multiple access (TDMA) super frame for transmission from afirst mote to a second mote and from the first mote to a third mote,wherein each transmission is allocated a distinct timeslot; whereintransmission from the first mote to the second mote is allocated a firsttimeslot, and wherein the second mote is a member of a first multicastgroup; determining if the third mote is a member of the first multicastgroup; if the third mote is the member of the first multicast group,reallocating a second timeslot allocated for transmission to the thirdmote based on a membership of the third mote to the first multicastgroup, wherein the reallocated second timeslot is shifted such that thesecond timeslot is adjacent to the first timeslot.
 14. The program ofclaim 13, further comprising: dynamically allocating a third timeslot ofthe TDMA super frame for transmission from the first mote to a fourthmote; determining if the fourth mote is a member of the first multicastgroup; if the fourth mote is the member of the first multicast group,reallocating the third timeslot based on a membership of the fourth moteto the first multicast group, wherein the reallocated third timeslot isshifted such that the third timeslot is adjacent to the second timeslot.15. The program of claim 13, wherein the motes wirelessly communicateusing an Institute of Electrical and Electronics Engineers (IEEE)802.15.4 protocol.
 16. The program of claim 13, wherein the moteswirelessly communicate over a low-power wireless personal area network.